The present invention relates to an apparatus for vaporizing and supplying an organometal compound used as a starting material for manufacturing an epitaxial thin film of a compound semiconductor according to the metal organic chemical vapor deposition (MOCVD) technique.
The MOCVD technique in which an organometal compound is employed as a starting material has become of major interest recently as a means for growing crystals of a compound semiconductor. According to the MOCVD technique, a crystalline thin film is formed or grows through the thermal decomposition of an organometal compound such as (CH.sub.3).sub.3 Ga or (CH.sub.3).sub.3 Al as a starting material followed by the deposition of the resulting vapor of the decomposed organometal compound onto the surface of a substrate. In particular, this technique makes it possible to form a thin film having a uniform thickness on a substrate having a large surface area and is excellent in mass-productivity and controllability of the film composition and thickness or the like.
When an organometal compound is used in the MOCVD technique, a carrier gas such as hydrogen gas is bubbled through the organometal compound contained in a container so that it comes in contact with the compound to give a carrier gas stream saturated with the vapor of the organometal compound having a desired temperature and the carrier gas is guided to a chamber for growing crystals in which a crystalline thin film grows on a substrate.
FIG. 10 shows a conventionally known apparatus for vaporizing and supplying an organometal compound. In FIG. 10, the numerical order 1 represents a container for a carrier gas such as H.sub.2 gas, 2 represents a valve for reducing pressure, 3 represents a massflow controller (MFC) for controlling the massflow of the carrier gas, 4 represents a liquid organometal compound as a starting material for epitaxial growth and 5 represents a cylinder cabinet which is filled with such a liquid organometal compound 4. The numerical order 6 represents a constant temperature oven, 7 represents an inlet valve and 8 represents a tube for introduction (a dipping tube) which serves to introduce the carrier gas into the lower portion of the cylinder cabinet 5 through an inlet thereof. The numerical order 9 represents an outlet valve and 10 represents a needle valve. The numerical order 11 represents a crystal growth chamber for growing crystals, 12 represents a heater and 13 represents a substrate.
Employing this apparatus for manufacturing a thin film of a semiconductor, first the temperature of the constant temperature oven 6 is correctly controlled to thus determine the vapor pressure of the organometal compound 4. Then the carrier gas of which massflow has been precisely controlled by the massflow controller 3 is introduced into the cylinder cabinet 5 by opening the valve 7 and thereafter the valve 9 is opened to guide the carrier gas containing the vaporized organometal compound in a desired concentration towards the crystal growth chamber 11. Thus, an epitaxial thin film of a semiconductor resulting from the organometal compound grows on the surface of the substrate 13.
However, the epitaxial film thus prepared sometimes has a composition which deviates from a desired composition and non-uniform electrical properties. It is assumed that such phenomena occur because the starting gas for the epitaxial film is not supplied to the crystal-growth chamber 11 in a constant feed rate or flow rate.
It is assumed that the cause of such unstable supply of the starting gas can be attributed to unstable flow rate of the starting gas per unit time, in the foregoing conventional apparatus. It is likewise considered that the supply of the starting gas cannot precisely be controlled since in the conventional apparatus, quick switching between the supply of the starting gas and the interruption thereof cannot be attained.
The optimum supply of the starting gas varies depending on the thickness of the intended epitaxial film and, therefore, it is preferable that the flow rate thereof be simply changed. However, in the aforementioned apparatus, it takes a long time period till a stable flow rate is attained after changing the flow rate of the starting gas. This leads to a low workability.
Moreover, when an epitaxial film comprising mixed crystals of a plurality of elements such as those comprising three or four elements is formed in the foregoing apparatus, the use of constant temperature ovens corresponding to the number of the elements is required in the system for supplying starting gas. As a result, the size of the whole system is enlarged substantially.